The manufacture of pressed bodies, in particular of tablets, on the basis of zeolites and binders is known. Thus, according to JP-A-61 15 5216, zeolite tablets are prepared by mixing a zeolite, a binder and a lubricant, and extruding the mixture. These are tablets with the same dimensions in all directions.
From JP-A-56 06 3818 the manufacture of zeolite tablets for use as gas-adsorbents is known. Powdered and dried (at 105 to 110° C.) zeolite is mixed with 8.1% by weight of bentonite powder and the mixture is kneaded with a 4% aqueous urea solution. The mixture is then tabletted, dried and calcined at 510° C. The increase of the compressive strength of the tablet is caused by the urea content.
From JP-A-55 16 5144 it is known to knead zeolite powder for cooling aggregates in powder form with bentonite and water. The mixture is extruded to form round particles with a diameter of 0.8 to 10 μm.
According to JP-A-55 10 4913, zeolite in the Na-form is mixed with 25% by weight clay, kneaded with water, extruded, calcined at 650° C., immersed in a calcium chloride solution, washed, dried at 110° C. and activated at 400° C. The tablets are used as drying agents.
According to JP-A-46 03 2572, zeolite powder is mixed with kaolin and Na- (or NH4-)-hydroxyethyl cellulose, shaped, dried and calcined at 650° C., in order to increase the strength of the zeolite tablets.
According to JP-A-21 44 121, deodorants are obtained by extruding zeolite powder or granules with calcium chloride or bentonite and water. The mixture is then tabletted and calcined.
According to JP-A-63 21 8234, drying agents are prepared by extruding a mixture of microporous particles (e.g. gypsum, cement, ceramic powder) and an inorganic or organic filler, such as CaCl2, LiCl, Bentonite, zeolites, PVA or other water-soluble polymers. The mixture is tabletted and subsequently hardened.
According to JP-A-60 132 643, zeolite tablets are prepared as drying agents using 20% sepiolite as a binder. The mixture is kneaded with water, tabletted, dried at 150° C. and calcined at 550° C. The tablets have an improved drying efficiency as compared to bentonite tablets.
The prior art tablets are unsuitable for utilization in a narrow space and under mechanical-stresses, because they are too thick and too heavy and have too low a sorption capacity for noxious gases and vapors (based on mass and surface). With the methods and mixtures according to the prior art, brittle pressed bodies were obtained which crumble, in particular after calcining.
It is known that electroluminescent devices function over a long period of time only if a drying agent is present. Such drying agents are required because of the sensitivity of the electrodes, in particular the cathodes, to moisture (cathodes generally consist of Ca or Mg alloys). Therefore in use, these devices are sealed as efficiently as possible under a protective gas.
The use of a moisture absorber in an electroluminescent device is disclosed in EP 500 382 A2. The moisture absorber is in the form of a powder or small spheres applied to a black silicone resin coating. According to the preferred embodiment, the drying agent is contained in a gas-permeable bag.
Likewise, U.S. Pat. No. 5,882,761 discloses the use of a drying agent in an electroluminescent device. The preferred drying agent is BaO. See also U.S. Pat. No. 5,591,379 which discloses a coating for use with microelectronic devices comprising a desiccant powder blended with a binder.
The sorbents known from the above-mentioned documents have the disadvantage that they can only absorb water vapor. The cathodes, however, can be degraded by other gases as well, which gases can be formed when the epoxy resin used for sealing is cured (ammonia, liquid amines). In addition, the presence of oxygen may result in a failure of the luminescent elements (oxidation of the cathode).
The present invention is based on the need to provide plate-shaped pressed bodies (wafers) comprising or formed from an inorganic sorbent and an inorganic binder which are very thin (less than 700 μm). In spite of being thin, these bodies have high strength and may be incorporated in particular into electronic elements which only have limited space available. These bodies are also shock resistant for uses such as electronic displays in automobiles and mobile telephones.
The present invention satisfies this need by providing plate-shaped pressed bodies (wafers) comprising or formed from an inorganic sorbent and a binder, having a thickness of less than 700 μm, which are obtainable by a process comprising the steps of compressing a mixture comprised of at least one inorganic sorbent, at least one binder and optionally water and a compression aid, at a pressure of at least 70 MPa; wherein the mixture has a weight ratio of dry sorbent to dry binder between about 4 and 0.7 and a water content of the mixture, measured at 160° C., between about 8 and 20%; and calcining the resulting pressed bodies at temperatures of at least 500° C., until the water content is substantially removed.